Michael Scanlon, a former business partner of the lobbyist Jack Abramoff and a former top aide to Rep. Tom DeLay, pleaded guilty on Monday to conspiring to bribe a member of Congress and other public officials. He agreed to repay $19.6 million to his former Indian tribe lobbying clients.

Scanlon acknowledged in a plea agreement that he and Abramoff, identified in the court papers as “Lobbyist A,” agreed to make lavish gifts to public officials, including all-expense-paid trips to Europe and the Super Bowl, in exchange for official actions.

Federal law enforcement officials portrayed the plea bargain, under which Scanlon faces up to five years in prison, as an important development in the larger criminal investigation of Abramoff, who has been under scrutiny by a grand jury here for more than a year.

The investigation, which initially centered on accusations that Abramoff had defrauded Indian tribe casinos of tens of millions of dollars in lobbying fees, has created alarm on Capitol Hill, where the lobbyist and his junior partner, Scanlon, claimed friendships among the Republican leaders of Congress.

Prosecutors have not named any of the public officials who were the targets of Scanlon’s scheme.

But court papers in the case filed Monday and last week singled out one member of Congress — “Representative No. 1” — as a focus of Scanlon’s illegal lobbying, asserting that the lawmaker accepted gifts, including a 2002 golf trip to Scotland and regular meals at Abramoff’s restaurant, “in exchange for a series of official acts and influence.”

Rep. Bob Ney, an Ohio Republican and chairman of the House Administration Committee, has acknowledged that he is the lawmaker, while saying there was no quid pro quo with Abramoff or Scanlon. Ney, who was subpoenaed this month by the grand jury investigating Abramoff, has said he was “duped” by the lobbyists.

Brian J. Walsh, the lawmaker’s spokesman, said, “All this plea agreement shows is that Scanlon had a deliberate, secret and well-concealed scheme to defraud many people and, it appears, unfortunately, that Rep. Ney was one of the many people defrauded.”

Scanlon, 35, a longtime Republican operative in the capital, said little during the hearing Monday in U.S. District Court here.

“Guilty, your honor,” he replied calmly when asked by Judge Ellen Segal Huvelle for his plea. Under the agreement with the Justice Department, Scanlon pleaded guilty to one count of conspiracy to violate a series of criminal laws, including those against bribery, and pledged to cooperate with the Justice Department’s investigation of Abramoff and others.

At a news conference after the hearing, Scanlon’s lawyer, Plato Cacheris, stood alongside his client and said Scanlon was “obviously regretful” about the fraud committed against the Indian tribes, which paid Abramoff and Scanlon more than $80 million in fees.